Alex Rider is such a likable character. This is the last book of the series.
Jack become more than a housekeeper, she is a parent and seem to be the only adult to have Alex's best interest in mind.
The writer has killed off this character to make us cry. Well done as a plot twist. But really too cruel for the character Alex who through our active imagination not only exist in our mind, but has feelings too.
So in this one, Alex get a hint that Jack might not be dead after all, but was held captive. That's would be the only reason she hasn't come back to Alex.
Alex is now living with Sabrina and her parents. They broken up and the relationship changed from couple to brother and sister.